Poly(arylene sulfide) resins (hereinafter abbreviated as “PAS resins”) represented by a poly(phenylene sulfide) resin (hereinafter abbreviated as “PPS resin”) are engineering plastics excellent in heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical properties, dimensional stability and the like. The PAS resins are commonly used as materials for resin parts in a wide variety of fields such as electrical and electronic equipments, automotive equipments and chemical equipments because they can be molded or formed into various kinds of molded products, films, sheets, fibers, etc. by general melt processing processes such as injection, molding, extrusion and compression molding.
As a typical production process of a PAS resin, is known a process in which a sulfur source and a dihalo-aromatic compound are subjected to a polymerization reaction in an organic amide solvent such as N-methyl-2-pyrrolidone (hereinafter abbreviated as “NMP”). However, the PAS resin has a defect that the amount of burr produced upon injection molding is great. The burr means portions of a molding material, which have flown out in interstices of a mold and solidified. Burr solidified in the form of a thin film or flake requires to be removed in a finishing step.
A process, in which a branched PAS resin or heat-crosslinked PAS resin is blended with a straight-chain PAS resin, has been proposed for inhibiting the occurrence of burr upon injection molding. In addition, some proposals have been made on a production process of the branched PAS resin. However, the conventional processes are difficult to cope with a high requirement level in recent years.
A PAS resin composition obtained by blending a crosslinked PAS resin having a melt viscosity of 5×105 to 1×109 P (5×104 to 1×108 Pa·s) as measured at a temperature of 310° C. and a shear rate of 5 sec−1 and exhibiting a gel form upon melting with an uncrosslinked and substantially linear straight-chain PAS resin, and improved in burr-producing tendency has heretofore been proposed in Japanese Patent Application Laid-Open No. 64-9266 (corresponding to U.S. Pat. No. 4,956,499; hereinafter referred to as “Article 1”).
Article 1 shows an Experimental Example in which a crosslinked PAS resin was produced by a process including polymerizing an alkali metal sulfide, a dihalo-aromatic compound and a polyhalo-aromatic compound having 3 or more halogen substituents in an organic amide solvent by two stages. More specifically, the crosslinked PAS resin is produced by a two-stage polymerization process including reacting sodium sulfide, p-dichlorobenzene and 1,2,4,5-tetrachlorobenzene in NMP, and then adding water and raising the temperature to continue the polymerization reaction. However, the crosslinked PAS resin obtained by this production process is a mixture of a small amount of a granular product and a great amount of a bulky product (see “Polymer Preparation Example B-1” in Article 1). A resin composition with such a crosslinked PAS resin blended with a straight-chain PAS resin is poor in molding and processing ability, and the surface properties of a molded product obtained therefrom are also poor.
In addition, Article 1 discloses a heat-crosslinked PAS resin obtained by subjecting a substantially linear straight-chain PAS resin to a heat treatment for a long period of time at a high temperature, thereby conducting high-temperature curing (see “Polymer Preparation Examples B-4 to B-6” in Article 1). However, a resin composition with the heat-crosslinked PAS resin blended with a straight-chain PAS resin has involved a problem that it corrodes a mold used in injection molding. The heat-crosslinked PAS resin is considered to contain a corrosive component caused by a thermal decomposition reaction or the like due to the heat treatment for a long period of time at a high temperature.
A process for producing an alkali metal sulfide, a dihalo-aromatic compound and a polyhalo-aromatic compound having 3 or more halogen substituents in an organic amide solvent by an improved two-stage polymerization process has heretofore been proposed in Japanese Patent Application Laid-Open No. 1-299826 (corresponding to U.S. Pat. Nos. 5,200,500 and 5,268,451; hereinafter referred to as “Article 2”). Article 2 discloses a process including reacting, in a first-stage polymerization step, the alkali metal sulfide, dihalo-aromatic compound and polyhalo-aromatic compound in a state that water is present in a proportion of 0.5 to 2.9 mol per mol of a charged alkali metal sulfide in the organic amide solvent, and controlling, in a second-stage polymerization step, the amount of water in such a manner that water is present in a proportion of 2.5 to 7 mol per mol of the charged alkali metal sulfide and raising the temperature to continue the polymerization reaction. According to the production process disclosed in Article 2, a highly crosslinked PAS resin can be obtained in the form of granules without forming a bulky product.
However, when the alkali metal sulfide, dihalo-aromatic compound and polyhalo-aromatic compound are reacted from the beginning of the polymerization, a branched PAS resin having a too high melt viscosity is easy to be provided. When the branched PAS resin having a too high melt viscosity is blended with a straight-chain PAS resin, defects such as hard spots (non-melt matter) and small depressions occur in a molded product to deteriorate its surface properties, and moreover the inhibitory effect on the occurrence of burr is also insufficient.
On the other hand, when the polymerization time in the second-stage polymerization step in the production process described in Article 2 is markedly shortened, a branched PAS resin having a low melt viscosity can be obtained. However, the branched PAS resin having a low melt viscosity obtained by such a process has great melt viscoelasticity tan δ, and the inhibitory effect on the occurrence of burr is poor even when it is blended with the straight-chain PAS resin, so that the surface properties of the resulting molded product are deteriorated.